1. Field of the Invention
The present invention relates to an outer lead forming apparatus for a semiconductor device and, more particularly, to a reduction in size and weight of a die of an outer lead forming apparatus.
2. Description of the Related Art
In this specification, a description of an apparatus will be made with reference to forming of the outer leads of a PLCC type semiconductor device.
Forming of the outer leads of a PLCC type semiconductor device includes the following steps.
A) Resin cut
Resin cut is the step of removing unnecessary portions of a resin used for packaging, which adhere to a lead frame (trimming).
B) Dam cut
A dam is a member, formed at an outer lead portion, for blocking a resin flowing along leads. For example, dams are formed to cross outer leads. Dam cut is the step of cutting the dams.
C) Lead cut
Lead cut is the step of separating the outer leads from the lead frame.
D) Lead end bending
Lead end bending is the step of slightly bending the distal ends of the separated outer leads. By slightly bending the distal ends of the leads, a curling direction, for example, in lead curling to be subsequently performed, is determined.
E) Lead pre-bending
Lead pre-bending is the step of pressing the outer leads by using a punch or the like and bending them by using a die, thus specifying bending portions of the outer leads.
F) Right-angle lead bending
Right-angle lead bending is the step of bending the outer leads at right angles from the bending portions specified in the step E) by using a punch and a die.
G) Lead curling
Lead curling is the step of curling the distal end portions of the outer leads. With this step, the leads are formed into J-shaped leads.
H) Tie-bar cut
A tie-bar is a member for connecting a lead frame and a semiconductor device to each other and preventing them from separating from each other. Tie-bar cut is the step of cutting this tie-bar
A conventional forming apparatus includes a progressive die using a lead frame as a belt plate and performs the above-described series of steps by using a single die.
FIG. 1 is a block diagram showing schematic arrangement of a conventional forming apparatus. Referring to FIG. 1, reference symbols A to H respectively denote step blocks corresponding to the steps A) to H). Reference numerals 102 on both the sides of the step blocks A to H respectively denote convey rails for feeding a lead frame forward. The lead frame is inserted in a progressive die 100 from a direction indicated by an arrow 108 in FIG. 1. FIG. 2 is a plan view of the lead frame. Two rows of sprocket holes 201 are respectively formed along two sides of the lead frame 200. Each row of the sprocket holes 201 is formed in correspondence with the position of a corresponding convey rail 102. The lead frame 200 is fed forward in the progress die 100 by using the sprocket holes 201. A predetermined number of semiconductor devices 202 molded with a resin or the like are mounted, in a row, on an area between the two rows of the sprocket holes 201. The lead frame 200 inserted in the progressive die 100 is processed in the following manner. In the step block A), resin burrs at an outer lead portion are removed. The lead frame 200 is then fed forward, and dams are cut in the step block B). In the step block C), outer leads are cut from the lead frame 200. In the step block D), the distal ends of the outer leads are bent. In the step block E), the outer leads are further bent. In the step block F), the outer leads are bent at nearly right angles. In the step block G), the distal ends of the outer leads are curled. In the step block H), a tie-bar is cut to separate the semiconductor device 202 from the lead frame 200. The separated semiconductor device 202 is pushed by a push rod in a direction indicated by an arrow 112 so as to be fed to an unloader 114.
There are some merits in the forming apparatus having the above-described arrangement. For example, the manufacturing process can be simplified by using the progressive die and integrating all of the steps including the steps of cutting the lead frame 200 (to be referred to as punching steps hereinafter) as in the step blocks A), B), C), and H), and the steps of outer leads (to be referred to as bending steps hereinafter) as in the step blocks D), E), F), and G).
If, however, the punching steps and the bending steps are integrated, the size of the die is undesirably increased. A typical size is: length: 500 to 600 [mm]; and depth: 350 [mm]. Consequently, the die has a weight of about 80 to 100 [kg], which is too heavy for one operator to carry, thus requiring a cumbersome operation for maintenance.
In addition, a distance L between the step blocks A) to H) is determined by an arrangement pitch D between the semiconductor devices 202 on the lead frame 200. The arrangement pitch is considerably reduced to increase the yield of semiconductor devices per lead frame. For example, when PLCC type semiconductor devices are to be processed, the arrangement pitch D is set to be about 30 to 50 [mm]. Consequently, the distance L between the step blocks A) to H) is reduced. In accordance with the reduced distance L, the bending step is performed by butting a punch into a U-shaped die, i.e., so-called "butt bending". FIGS. 3A and 3B show an example of butt bending. FIGS. 3A and 3B show the step F). As shown in FIG. 3A, the semiconductor device 202 is placed between a punch 204 and a U-shaped die 206. The punch 204 is then butted into the U-shaped die 206, as shown in FIG. 3B. In this method, however, since the die 206 and leads 208 are brought into contact with other, the leads 208 tend to be damaged. Damage to the leads 208 causes a deterioration in quality of the semiconductor device. In addition, if the leads 208 are damaged, a plating (solder plating) layer of each lead is peeled off, and peeled pieces of the plating layer are left in the die. For this reason, maintenance and inspection and the like must be performed frequently. Furthermore, since the force of friction between the leads and the die is large, a gap is formed between the seal resin of the semiconductor device and the proximal end portion of each lead.
Moreover, since the bending steps and the punching steps are integrated, an unbalanced force is applied to the die. For example, in the punching steps, a large force is required to punch a lead frame. In the bending steps, however, such a large force is not required. The pressure to be applied to the die is determined in accordance with the punching steps which require a large force. For this reason, an excessive pressure is applied in the bending steps. This makes the problem of damage to leads more serious.